In photoelectric incremental length and angle measuring instruments, it is well known to provide reference marks, to fix and reproduce a defined zero point. For example, U.S. Pat. No. 4,677,293 describes the makeup of a reference mark for generating a reference pulse. Phase gratings are used as graduation markings to diffract the radiation of a light source and direct it to detectors. The evaluation of push-pull or pseudo-push-pull signals furnishes a reference pulse, which is relatively independent of the intensity of illumination (and within certain limits of changes in spacing as well). The pulse width of the reference pulse is substantially determined by the widths of the graduation markings on the graduation-carrying substrate and by the widths of the gaps in the scanning plate.
However, the device described in this patent has the disadvantage that diffraction effects prevent it from producing reference pulses with very small pulse widths. These diffraction effects are apparent upon observation of casting shadows produced by the device. Consequently, the device is inadequate for use in high-resolution measuring systems due to its inability to produce small pulse width reference pulses necessary for such systems. Moreover, in such a device the scanner plate and the graduation-carrying substrate must be kept closely spaced apart from one another.
U.S. Pat. No. 4,778,273 discloses a reference pulse transducer that does not have the disadvantages present in the device disclosed in the '293 patent described above. The device disclosed in the '273 patent comprises a plurality of gratings with periodic graduations with different grating constants, from which signals with differing periods are derived, by the three-grating principle. This principle is more fully described in "Dreigitterschrittgeber-Photoelektrische Aufnehmer zur Messung von Lageanderungen" [Three-Grating Incremental Transducers--Photoelectric Pickups for Measuring Positional Changes], dissertation by J. Willhelm, Technische Universitat Hannover [Technical University, Hanover].
The signals produced in the '273 device may be combined in accordance with a Fourier series. As a result, reference pulses of very small width can be produced, which hardly depend on the spacing between the scanner plate and the graduation-carrying substrate. However, the device described in the '273 patent has the disadvantage of furnishing a periodic train of pulses instead of furnishing only a single reference pulse. Consequently, additional provisions must be made to make these pulses distinguishable from one another. Since the pulse spacing cannot be increased arbitrarily, considerable effort and expenditure are needed to select one pulse.